Yea, just realised my post sort of had nothing to do with the original topic. Ahem. 
In terms of RPing this out (assuming that the multi-killing going on is legal and non-trashy for the killer, such as the victim keeps throwing themself at the killer) I think a cool way would be to use the fact that you don't know they've been granted another life.
Foolish Victim walks in.
You: "What? My dastardly scheme failed!"
Foolish Victim: "What do you mean?"
You: "My attempt at taking your life was evidently futile!"
Foolish Victim: "Gonna get'cha now!"
** Fight, victim dies again, you walk off **.
You walk into a room, and see Foolish Victim:
You: "Does this never end? Am I such a failure? Perhaps I should take up knitting!"
Pretty poor example I suppose. My views on multi-killing are dependant on individual cases. I think it was L-A who said something along the lines of, "If someone kills you, then patrols your EQ spots to keep you down, why aren't you doing it to them?".
Looking at the bigger picture, multi-killing, to a certain degree/limit is within the rules. If a certain character in particular, such as say, being Caballed and some non-Caballed char attacks my Cabal, I'm going to try and get them a couple of times, and make statement. That char who constantly interrupts your progress to fifty, killing party members and attacking you whenever they log on? Reach fifty, and turn the tables.
I think there needs to be a justifiable reason for it, although most characters RP would allow them to come up with a reason anyway, and that's where the main problem lies. A dark-knight could kill a Paladin, then kill him again, then kill him again, and merely say, "Oh, I'm so nasty, I want to see this purifier suffer." or whatever.
My concluded thoughts?
If they do it to me, I'm going to play ball.
If they are really going out of their way to give me grief, verbal or physical, I might decide to play ball dependant on the intensity of the situation.
If they are doing something that I, as a character, would find extremely disrespectful, such as an uncaballed attacking my Cabal, I'm going to play ball.
By playing ball, again, I'm not talking excessive overkill, or for that fact it doesn't even mean killing them as soon as their PK shield drops. It could well be over the span of a few real-life days or whatever.
I think I've rambled on a bit too much here.
Dey